revlog: move loading of index data into own method
This will allow us to "reload" a revlog instance from a rewritten
index file, which will be used in a subsequent commit.
Differential Revision: https://phab.mercurial-scm.org/D4868
{
"conduit_uri": "https://phab.mercurial-scm.org/api",
"repository.callsign": "HG",
"arc.land.onto.default": "@",
"base": "hg:.^"
}